CVE-2026-89581: bpf, x86: Fix per-CPU address resolution into an extended register

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

bpf, x86: Fix per-CPU address resolution into an extended register

The destination of the per-CPU address MOV is encoded in ModRM.reg,
which is extended by REX.R, but the REX prefix is built with
add_1mod(), which sets REX.B. REX.B extends ModRM.rm and SIB.base, and
this instruction addresses memory as disp32 with no base, so the bit
has no effect at all and the high register bit is simply lost.

Every is_ereg() destination therefore resolves to the wrong register,
picking whichever one shares the low three bits:

  R5 -> RAX    R7 -> RBP    R8 -> RSI    R9 -> RDI

With BPF_REG_5, whose reg2hex is 0, the emitted

  65 49 03 04 25 <off>	add %gs:<off>,%rax

adds the per-CPU offset to RAX rather than R8. The destination keeps
the unadjusted address and RAX is clobbered, so the program goes on to
dereference a pointer that was never made per-CPU:

  BUG: unable to handle page fault for address: 0000607e386a8894
  RIP: bpf_prog_707837aafd2aa9ae_update_percpu_data+0x93/0xc9
  Call Trace:
   __bpf_prog_test_run_raw_tp+0x2dc/0x7d0
   __flush_smp_call_function_queue+0x1e9/0xc80
  Kernel panic - not syncing: Fatal exception in interrupt

R5 is the mildest of the four, aliasing a scratch register and faulting
at the store. R7 aliases RBP and would corrupt the frame pointer, R8
and R9 alias the argument registers.

Use add_2mod() so the register goes through REX.R, matching how
add_2reg() places it in ModRM.reg and how emit_priv_frame_ptr()
hardcodes 0x4c for the same instruction with R9. Encodings for the
non-extended registers are unchanged.

Problem showed up when trying to resurrect BPF_GCC CI (selftests built
with BPF_GCC).

This has gone unnoticed because clang reloads the address into R1
before each per-CPU access, so the destination is never an extended
register. GCC keeps several per-CPU addresses live at once, and
test_progs-bpf_gcc panics the kernel in global_percpu_data/init, where
the address of a .percpu variable ends up in R5.

The Linux kernel CVE team has assigned CVE-2026-89581 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.10 with commit 7bdbf7446305cb65c510c16d57cde82bc76b234a and fixed in 6.12.109 with commit 638bc3aada8ecdece184d5c15b100d489c9cccd7
	Issue introduced in 6.10 with commit 7bdbf7446305cb65c510c16d57cde82bc76b234a and fixed in 6.18.50 with commit 6886642414f59f928728802dc2c972a9859e6310
	Issue introduced in 6.10 with commit 7bdbf7446305cb65c510c16d57cde82bc76b234a and fixed in 7.2.4 with commit 6a19b18d458881bf3269a357cc6dc6db5eef4369
	Issue introduced in 6.10 with commit 7bdbf7446305cb65c510c16d57cde82bc76b234a and fixed in 7.3-rc1 with commit 5bbbce02e500d47d8e259a45be5a7be9741d0533

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-89581
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	arch/x86/net/bpf_jit_comp.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/638bc3aada8ecdece184d5c15b100d489c9cccd7
	https://git.kernel.org/stable/c/6886642414f59f928728802dc2c972a9859e6310
	https://git.kernel.org/stable/c/6a19b18d458881bf3269a357cc6dc6db5eef4369
	https://git.kernel.org/stable/c/5bbbce02e500d47d8e259a45be5a7be9741d0533